Light source

ABSTRACT

A light source to be powered by microwave energy, the source having: a body having a sealed void therein, a microwave-enclosing Faraday cage surrounding the body, a fill in the void of material excitable by microwave energy to form a light emitting plasma therein, and an antenna arranged within the body for transmitting plasma-inducing, microwave energy to the fill, the antenna having: a connection extending outside the body for coupling to a source of microwave energy; wherein the body is a solid plasma crucible of material which is lucent for exit of light therefrom, the plasma crucible is contoured with a contoured surface to reflect light internally within the lucent crucible material to exit in a particular direction, the Faraday cage is at least partially light transmitting for light exit from the plasma crucible in the particular direction.

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/671,088, filed Jan. 28, 2010 which is a national stage entryof and claims priority to PCT/GB08/03829 filed Nov. 14, 2008 which inturn claims priority to application numbers 0722548.5 filed Nov. 16,2007 (Great Britain), 0809471.6 filed May 23, 2008 (Great Britain),0814699.5 filed Aug. 12, 2008 (Great Britain) and 0814701.9 filed Aug.12, 2008 (Great Britain), the entire contents of each are incorporatedherein in their entirety.

The present invention relates to a light source for a microwave-poweredlamp.

It is known to excite a discharge in a capsule with a view to producinglight. Typical examples are sodium discharge lamps and fluorescent tubelamps. The latter use mercury vapour, which produces ultravioletradiation. In turn, this excites fluorescent powder to produce light.Such lamps are more efficient in terms of lumens of light emitted perwatt of electricity consumed than tungsten filament lamps. However, theystill suffer the disadvantage of requiring electrodes within thecapsule. Since these carry the current required for the discharge, theydegrade and ultimately fail.

We have developed electrodeless bulb lamps, as shown in our patentapplication Nos. PCT/GB2006/002018 for a lamp (our “'2018 lamp”),PCT/GB2005/005080 for a bulb for the lamp and PCT/GB2007/001935 for amatching circuit for a microwave-powered lamp. These all relate to lampsoperating electrodelessly by use of microwave energy to stimulate lightemitting plasma in the bulbs. Earlier proposals involving use of anairwave for coupling the microwave energy into a bulb have been made forinstance by Fusion Lighting Corporation as in their U.S. Pat. No.5,334,913. If an air wave guide is used, the lamp is bulky, because thephysical size of the wave guide is a fraction of the wave length of themicrowaves in air. This is not a problem for street lighting forinstance but renders this type of light unsuitable for manyapplications. For this reason, our '2018 lamp uses a dielectricwave-guide, which substantially reduces the wave length at the operatingfrequency of 2.4 Ghz. This lamp is suitable for use in domesticappliances such as rear projection television.

U.S. Pat. No. 6,737,809 describes: a light source to be powered bymicrowave energy, the source having:

-   -   a body having a sealed void therein,    -   a microwave-enclosing Faraday cage surrounding the body,        -   the body and the cavity defining a resonant waveguide,    -   a fill in the void of material excitable by microwave energy to        form a light emitting plasma therein, and    -   an antenna arranged within the body for transmitting        plasma-inducing, microwave energy to the fill, the antenna        having:        -   a connection extending outside the body for coupling to a            source of microwave energy.

We now believe that it is possible to coalesce the bulb and the waveguide into a single component. The object of the present invention is toprovide an improved lamp having such a coalesced bulb and wave-guide.

According to the invention there is provided a light source to bepowered by microwave energy, the source having: a body having a sealedvoid therein, a microwave-enclosing Faraday cage surrounding the body,the body within the Faraday cage being a resonant waveguide, a fill inthe void of material excitable by microwave energy to form a lightemitting plasma therein, and an antenna arranged within the body fortransmitting plasma-inducing, microwave energy to the fill, the antennahaving: a connection extending outside the body for coupling to a sourceof microwave energy; wherein: the body is a solid plasma crucible ofmaterial which is lucent for exit of light therefrom, the plasmacrucible is contoured with a contoured surface to reflect lightinternally within the lucent crucible material to exit in a particulardirection, the Faraday cage is at least partially light transmitting forlight exit from the plasma crucible in the particular direction, theFaraday cage including: metalisation on the contoured surface of theplasma crucible to enhance reflection and forming part of the Faradaycage or a complementary reflector complementary to the contouredsurface, positioned to reflect light back through the plasma crucibleand forming part of the Faraday cage and the arrangement being such thatlight from a plasma in the void can pass through the plasma crucible andeither pass directly from it via the cage or pass indirectly from it viathe cage after reflection at the contoured surface.

As used in this specification: “lucent” means that the material, ofwhich the item described as lucent, is transparent or translucent;“plasma crucible” means a closed body enclosing a plasma, the latterbeing in the void when the latter's fill is excited by microwave energyfrom the antenna.

Normally, the material of the crucible will be a solid, dielectricmaterial.

Whilst it can be envisaged that the solid plasma crucible could havevarying structures and compositions throughout its volume, particularlywhere it is comprised of more than one piece sealed together, we wouldnormally expect it to be substantially homogenous throughout its volume.

In the second embodiment described below of the parent U.S. applicationSer. No. 12/671,088 from which the present application has been divided,the plasma crucible is of circular cross-section and is dimensioned fora half wave to extend diametrically within it. This light source willnormally be used with its light being reflected in a particulardirection.

It is envisaged that the plasma crucible will be of quartz or sintered,transparent ceramic material, although other materials may also besuitable. In particular, the ceramic material can be translucent ortransparent. An example of a suitable translucent ceramic ispolycrystalline alumina and example of a transparent ceramic ispolycrystalline Yttrium Aluminium Garnet—YAG. Other possible materialsare aluminium nitride and single crystal sapphire.

The Faraday cage can be provided by coating the plasma crucible with athin layer of conductive, transparent material, such as indium, tinoxide. Alternatively the plasma crucible can be encased in a mesh ofconductive wire. Again the conductive mesh can be fused into thematerial of the plasma crucible, with plasma crucible material extendingoutside the mesh.

The antenna may extend into the plasma void, when of suitable materialto resist attack by the fill particularly where the plasma crucible hasa wall thickness that is small in comparison with distance within theplasma crucible from the Faraday cage at one side or end and to theother side or end. In this case, resonance can be establishedpredominantly within the void. Such an antenna can be a rod extendinginto the void, but is preferably a plate, typically a disc, arrangedtransversely of the length of the plasma crucible. The connection forthe antenna can extend sideways out of the plasma crucible in or closeto a plane of the antenna; or, as is preferred, it can extend axiallyout of the plasma crucible, transversely of a plane of the antenna.

Alternatively, the antenna can be a rod of conductive metal extendingwithin a re-entrant in the plasma crucible. Such re-entrant can be athin walled projection into the void, with the rod antenna actingsimilarly to the plate antenna just mentioned. The re-entrant can beparallel to a length of the void or transverse to it. As an alternative,where the void is small in comparison with distance within the plasmacrucible from the Faraday cage at one side or end and to the other, there-entrant can be along side the void, with resonance being establishedacross the plasma crucible, largely within the plasma crucible. In thiscase, the plasma crucible will have a dielectric constant greater thanthat of the ambient atmosphere and the wave length of the resonance willbe shorter than its free space wavelength.

Whilst the plasma crucible can be one or an integer multiple of onewavelength of resonant microwaves within the plasma crucible, it ispreferably one half of the wave length.

The fill material can be any of a number of elements known to emit lightfrom a plasma, either alone or in combination.

Preferably, the Faraday cage includes at least one aperture for locallyincreasing light transmission therethrough. Usually, the aperture willbe no bigger than one tenth of the free space wave length of themicrowaves in the crucible. Typically for operation at 2.45 GHz, theaperture would be no bigger than 1/10×12.24 cm, i.e. 12.24 mm and for5.8 GHz no bigger than 6.12 mm.

More than one aperture can be provided. For instance, where light istaken both axially and radially from the crucible, correspondinglypositioned apertures can be provided.

Provision of the apertured region allows radiation of more light fromthe light source than would be the case in its absence.

Preferably the lucent plasma crucible has:

-   -   a bore having a step and a counter-bore extending from the void        to a surface of the crucible and    -   a plug of lucent material in the counter-bore and sealed to the        crucible.

The step and the void can be formed by mechanical boring of the materialof the crucible or other forming means, such as casting.

Whilst it is anticipated that with compatible coefficients of thermalexpansion, as between artificial sapphire for the plug and lucentalumina for the crucible, the plug and crucible can be of differentmaterials, normally they will be of the same material, typically quartz.

Again the plug can be sealed with a fusible material between the plugand the crucible, such as frit, but in the preferred embodiment the plugand the crucible are sealed by fusing of their own material. For fusing,the crucible can be heated as a whole. However local heating confined tothe region of fusing is preferable. Typically this can be done with alaser.

The plug can be of the same depth as the step, in which case, the plugis flush with the surface of the crucible. However, the plug can beproud of the surface. These two alternatives are suitable where the voidis to be close to the surface of the crucible. In a third alternativewhere the void is to be deeper in the crucible, the plug is recessed. Inthis latter embodiment, the length of the counter-bore to the surfacecan be filled with a further plug of the same material fixed, but notnecessarily sealed, in the counter-bore, with the further plug flushwith the surface. This arrangement allows the void to be central in thecrucible and the crucible to appear—as regards its dielectricmaterial—to behave as a single solid body (with the central void).

Preferably the light source is combined into a lamp with a source ofmicrowaves and a matching circuit as a single integrated structure.

Whilst the microwave source can be a solid state oscillator andamplifier, in the preferred embodiment, in view of the output, thesource is a magnetron. Typically the power of the magnetron will be 1kW.

In the preferred embodiment, the matching circuit is a stub tuner,conveniently a three-stub tuner.

It should be noted that whereas usually light source of the inventionare expected to use for producing visible light, they are suitable forproducing invisible light as well, in particular ultra violet light.

To help understanding of the invention, various specific embodimentsthereof will now be described by way of example and with reference tothe accompanying drawings, in which:

FIG. 1 a side view of a light source in accordance with the invention ofparent U.S. application Ser. No. 12/671,088 in combination as a lampwith a microwave drive circuit;

FIG. 2 is the light source in the lamp of FIG. 1, shown on a largerscale;

FIG. 3 is a similar view of the stub tuner of the microwave drivecircuit of FIG. 1;

FIG. 4 is a scrap cross-sectional view of the junction between the lightsource and the stub tuner;

FIG. 5 is a view similar to FIG. 2 of an alternative light source, alsoin accordance with the invention of the parent U.S. application Ser. No.12/671,088;

FIG. 6 is a perspective view of a plasma crucible of another lightsource of the invention of the parent U.S. application Ser. No.12/671,088;

FIG. 7 is a perspective view of a lucent plasma crucible for a lightsource of the invention of the present application which has beendivided from the parent U.S. application Ser. No. 12/671,088;

FIG. 8 is a cross-sectional side view of the further light source of thepresent invention, including a portion of a matching circuit and anadapter for the plasma crucible;

FIG. 9 is a perspective view of a lucent plasma crucible for anotherlight, again in accordance with the invention of the parent U.S.application Ser. No. 12/671,088;

FIG. 10 is a diagrammatic view of a microwave powered lamp including thelucent plasma crucible of FIG. 9;

FIG. 11 is a perspective view of a further lucent plasma crucibleaccording to the invention of the parent U.S. application Ser. No.12/671,088 for a microwave powered lamp;

FIG. 12 is a diagrammatic view of a microwave powered lamp including thelucent plasma crucible of FIG. 11;

FIG. 13 is a view similar to FIG. 11 of another lucent plasma crucibleaccording to the invention of the parent U.S. application Ser. No.12/671,088 and

FIG. 14 is a view similar to FIG. 12 of the crucible of FIG. 13 only.

Referring to FIGS. 1 to 5 of the drawings, a lamp of the invention ofparent U.S. application Ser. No. 12/671,088 comprises a light source inthe form of a light emitting resonator 1, a magnetron 2 and a stub tuner3. A reflector 4 is fitted at the junction of the light source and thestub tuner, for directing the light in a generally collimated beam 5.

The light emitting resonator comprises an crucible 11 formed of innerand outer envelopes 12, 13 of quartz. These are circular cylindricaltubes 14, 15, with respective end plates 16, 17. A Faraday cage in theform of a tungsten wire mesh 18, of a mesh size to exhibit a groundplane to microwaves within the resonator, is sandwiched between thetubes and the end plates respectively. Each envelope, comprised of itstube and end plates is hermetic. An earth connection 18′ extends fromthe mesh to the outside of the envelope.

The length axially of the crucible between the wire mesh sandwichedbetween the end plates is λ/2 for the operating microwave frequency. Atone end of the crucible, a molybdenum drive connection 19 extends to atungsten disc 20. This is arranged transverse the axis A of the crucibleat 1/16λ from the mesh at its end of the crucible. The crucible isfilled with excitable plasma material, such as a dose of metal halide ina rare earth gas.

The disc acts as an antenna and is driven by the magnetron 2, via thematching circuit 3. The matching circuit is an air wave guide 32 ofaluminium having the output antenna 22 of the magnetron as its input.The output antenna 33 of the matching circuit is a disc such as theresonator antenna disc and is connected to a connection 34 passing outof the matching circuit and insulated therefrom by an insulating bush35. The matching circuit has three tuning stubs 36, 37,38. These arearranged as λ/4, configuring the matching circuit as a stub tuner.

The matching circuit has flanges 39, 40 at its ends via which it isconnected to the magnetron and the light source. The end of the latteris cemented 41 into a holder 42 of ceramic material. This has bores 43at the same PCD as bores 44 in the flange 40 of the matching circuit andto which it is fastened by screws 45. A spacer ring 46 spaces thematching circuit and the holder, allowing the stub tuner and lightsource connections 34,19 to be coaxial and connected to each other by aclip 47. The reflector 4 is also carried on the screws between theholder 42 and the spacer 46. The earth connections 18′ are alsoconnected to the screws 45.

FIG. 5 shows an alternative light-emitting resonator, also having innerand outer envelopes of quartz with a ground plane mesh between them. Inplace of the disc antenna 20, a rod like antenna 120 extends in are-entrant sleeve 121 of quartz, on the central axis of the envelopes.This arrangement completely isolates the antenna from the fill contentsof the crucible, which is of advantage where the fill is particularlyaggressive.

In operation, the magnetron, typically rated at 1 to 5 kW, insertsresonant microwave radiation via the stub tuner and the antenna 20 or120 into the crucible. This forms a mixed dielectric resonant cavity.The resonance builds the intensity of the electric fields in the cavitysuch that the fill forms a plasma which radiates light. Typically themode of resonance will be TE101. Further modes of resonance are alsopossible.

Typically at 5.8 GHz, the axial length of the crucible between the meshat opposite ends and allowing for 1.5 mm of individual envelope wallthickness is 72 mm and the diameter is 31 mm. It will be appreciatedthat such a size, whilst too large for most domestic uses, is entirelysuitable for illuminating larger environments.

The stub tuner can have internal dimensions of 114×40×20 mm. The stubsare set of the median plane by 1/16λ. This has been found to beadvantageous.

It is possible to replace the quartz material of the plasma cruciblewith transparent ceramic, in which case the connector in contact withthe ceramic can be of niobium. Further in place of the mesh within thecrucible walls, the crucible can be coated with an indium tinoxide—ITO—conductive coating.

As shown in FIG. 6, the light source can be constructed with asub-assembly of an molybdenum end cap 51 having a molybdenum rod 52brazed 53 into it and carrying a tungsten antenna 54. The edge 55 of thecap is let into a neck 56 of the quartz end cover 57 of the crucible.This sub-assembly is sealed on the cylindrical body 58 and opposite end59 of the crucible at a seal 60. The cover 57 has a charge tube 61,through which the excitable material charge and noble gas fill can beintroduced. The tube is sealed off. The Faraday cage 62 is provided inthe form of an ITO coating.

Turning on now to FIGS. 7 & 8, a lamp of the invention of the presentdivisional application will now be described. It has a solid plasmacrucible 101 of polished quartz, with a flat front face 102 and aparabolic rear face 103. The front face is coated with indium tin oxide104 to render it electrically conductive, yet transparent. In electricalcontact with the ITO layer, is a platinum layer 105 on the parabolicrear. These two layers together form a Faraday cage around the quartzplasma crucible.

At the focus of the parabola and aligned with its central axis is a void106, filled with microwave excitable material 107, typically indiumhalide in xenon. The void is a bore in the quartz, that is sealed bymeans of a plug 108, the plug having been fused in place without othermaterial by laser sealing.

Alongside the void is a receptacle 109 in the quartz for a metal rodantenna 110. This is connected directly to the output 111 of a matchingcircuit such as the circuit 3. An adaptor plate 112 of the circuit has acontour 113 complementary to that of the rear face of the quartz plasmacrucible. A fastening ring 114 pulls the quartz into contact with theend plate, for grounding of the Faraday cage.

On propagation of microwaves from the matching circuit, resonance is setup in the quartz plasma crucible and a plasma is established in thevoid. Light is emitted from the halide in the void. This either leavesthe plasma crucible directly through the front face 102 or is reflectedby the platinum layer 105 at the parabolic back face 103 forwards toexit the front face.

Typically, the quartz plasma crucible is 49 mm in diameter for 2.4 GHzmicrowaves and 31.5 mm for 5.8 GHz. In either case, the void is 5 mm indiameter and the plug is 8 mm long, leaving a 10 mm long void. Theantenna receptacle 109 is 2 mm in diameter, being 5 mm eccentric fromthe void, which is on the central axis of the plasma crucible.

It should be noted that by comparison with prior electrodeless lampsusing small bulbs in opaque wave guides, where the light exit isrestricted to the diameter of the bulb, not only can light exit from thefull front face of the wave guide, which is significantly larger thanthe diameter of the plasma void 106, sideways and rearwards propagatinglight is reflected forwards and out of the lamp.

Referring to FIGS. 9 and 10, a lamp 201 of parent U.S. application Ser.No. 12/671,088 comprises an oscillator 202 and amplifier 203 togetherforming a source of microwave energy, typically operating at 2.45 or 5.8GHz or other frequencies within an ISM band. The source passes themicrowaves via a matching circuit 204 to an antenna 205 extending into are-entrant 206 in a lucent, plasma crucible 207. This is of quartz andhas a central void 208 containing a fill of noble gas and a microwaveexcitable material, which radiates light when excited by microwaves. Thequartz being transparent, light can leave it in any direction, subjectto the constraints provided by the Faraday cage described below.

The crucible is a right circular cylinder, 63 mm long and 43 mm indiameter. Centrally in the crucible, the void is 10 mm long and 3 mm indiameter. The re-entrant is co-axial with the void, being 2 mm indiameter and 10 mm long.

A Faraday cage 209 surrounds the crucible and comprises:

-   -   a light reflective coating 210, typically of silver with silicon        monoxide, across the end surface 211 having the antenna        re-entrant,    -   an indium tin oxide (ITO) deposit 212 on the end surface 214 and    -   a conductive, chemical-vapour-deposited mesh 215 on the        cylindrical surface 216, the mesh having fingers 217 which        extend onto the ends, for electrical interconnection of the        elements 210, 212 & 215. The lines of the mesh are 0.5 mm wide        and set at a pitch of 6.0 mm.

The Faraday cage is earthed by being received in a recess 218 in ahousing 219.

The ITO deposit has an un-plated 12 mm aperture 220 centrally placed inthe end face 214, whereby light 221 from the end of the plasma discharge222 in the void can pass directly out of the lucent plasma crucible,without attenuation in by the Faraday cage. Much light also passes outvia the Faraday cage, although attenuated to an extent.

It should be noted that Faraday cage can be formed entirely of wire meshformed around the crucible, with an aperture in line with the void.

Referring to FIGS. 11 & 12 of the drawings, a lamp 301 of parent U.S.application Ser. No. 12/671,088 comprises an oscillator and amplifiersource 302 of microwave energy, typically operating at 2.45 or 5.8 GHzor other frequencies within an ISM band. The source passes themicrowaves via a matching circuit 303 to an antenna 304 extending into are-entrant 305 in a lucent, plasma crucible 306. This is of quartz andhas a central void 307 containing a fill of noble gas and a microwaveexcitable material, which radiates light when excited by microwaves. Thequartz being transparent, light can leave it in any direction, subjectto the constraints provided by the Faraday cage described below.

The crucible is a right circular cylinder, 63 mm long and 43 mm indiameter. Centrally in the crucible, on its central longitudinal axis A,the void is 10 mm long and 3 mm in diameter. The re-entrant is co-axialwith the void, being 2 mm in diameter and 10 mm long.

A Faraday cage 308 surrounds the crucible and comprises:

-   -   a light reflective coating 310, typically of silver with silicon        monoxide, 309 across the end surface 310 having the antenna        re-entrant, the plating being reflective for reflecting light        from a plasma in the void out of the crucible,    -   an indium tin oxide (ITO) deposit 311 on an end surface 312 of        the crucible, the ITO coating passing light from the plasma and    -   a conductive, chemical-vapour-deposited mesh 314 on the        cylindrical surface 315, the mesh having fingers 316 which        extend onto the ends, for electrical interconnection of the        elements 309, 311 & 314. Light from the plasma can exit the        crucible between the mesh lines.

The Faraday cage is earthed by being partially received in a recess 317in an aluminium housing 318.

The end surface 312 has a bore 321 for receiving a plug 322, of the samematerial as the crucible, namely quartz. The bore forms a step 324 onwhich the plug is located with its outer surface 325 flush with thesurface 312 and to which the central void extends. The plug is sealed tothe seat by laser sealing at the corner between bore 321 and the step324.

Turning now to FIGS. 13 and 14, the light source there shown of parentU.S. application Ser. No. 12/671,088—without any of its drive antenna,Faraday cage nor a microwave source and matching circuit shown islargely similar to that of FIGS. 11 & 12. The crucible 406 has a centralvoid 407, which is truly at the centre of crucible, both longitudinallyand diametrically whereas the void 307 is diametrically central only.The bore 421 extends deeper into the crucible with the plug 422 being ofthe same thickness and resting on the step 424 at the junction of thebore and the void. The plug 422 is laser sealed in the same way as theplug 322.

Outside the plug 422, in the bore 421 is a further plug 431 extendingfrom the plug 422 to the surface 412 of the crucible. Thus for thepurposes of microwave resonance, the crucible is a continuous piece ofmaterial with the dielectric constant of quartz.

The invention is not intended to be restricted to the details of theabove described embodiments. For instance, the two plugs 422 and 431could be provided as a single whole.

Although the claimed subject matter has been fully described inconnection with examples thereof and with reference to the accompanyingdrawings, it is to be noted that various changes and modifications willbecome apparent to those skilled in the art. Such changes andmodifications are to be understood as being included within the scope ofthe present disclosure as defined by the appended claims.

The invention claimed is:
 1. A light source to be powered by microwaveenergy, the source having: a body having a sealed void therein, amicrowave-enclosing Faraday cage surrounding the body, a fill in thevoid of material excitable by microwave energy to form a light emittingplasma therein, and an antenna arranged within the body for transmittingplasma-inducing, microwave energy to the fill, the antenna having: aconnection extending outside the body for coupling to a source ofmicrowave energy; wherein: the body is a solid plasma crucible ofmaterial which is lucent for exit of light therefrom, the plasmacrucible is contoured with a contoured surface to reflect lightinternally within the lucent crucible material to exit in a particulardirection, the Faraday cage is at least partially light transmitting forlight exit from the plasma crucible in the particular direction, theFaraday cage including: metalisation on the contoured surface of theplasma crucible to enhance reflection and forming part of the Faradaycage or a complementary reflector complementary to the contouredsurface, positioned to reflect light back through the plasma crucibleand forming part of the Faraday cage and the arrangement being such thatlight from a plasma in the void can pass through the plasma crucible andeither pass directly from it via the cage or pass indirectly from it viathe cage after reflection at the contoured surface.
 2. A light source asclaimed in claim 1, wherein the plasma crucible is of a plurality ofpieces sealed together.
 3. A light source as claimed in claim 1, whereinthe plasma crucible is homogenous.
 4. A light source as claimed in claim1, wherein the plasma crucible is of circular cross-section and isdimensioned for a half wave to extend diametrically within it.
 5. Alight source as claimed in claim 1, wherein the plasma crucible is ofquartz or polycrystalline alumina or polycrystalline Yttrium AluminiumGarnet or aluminium nitride or single crystal sapphire.
 6. A lightsource as claimed in claim 1, wherein for light exit from the plasmacrucible in the particular direction the Faraday cage is of or includesa thin layer of conductive, transparent material and/or a mesh ofconductive wire and/or reticular metal sheet.
 7. A light source asclaimed in claim 6, wherein the conductive mesh or reticular sheet isfused into the material of the plasma crucible.
 8. A light source asclaimed in claim 6, wherein the Faraday cage includes at least oneaperture for locally increasing light transmission therethrough, theaperture preferably being is no bigger than one tenth of the free spacewave length of the microwaves in the crucible.
 9. A light source asclaimed in claim 1, wherein the antenna is a rod or wire of conductivemetal extending within a re-entrant in the plasma crucible and theconnection is an integral extension of the antenna rod or wire.
 10. Alight source as claimed in claim 1, wherein the void is small incomparison with a distance within the plasma crucible from the Faradaycage at one side or end and to the opposite side or end and there-entrant is along side or in line with the void.
 11. A light source asclaimed in claim 1, wherein the lucent plasma crucible has: a borehaving a step and a counter-bore extending from the void to a surface ofthe crucible and a plug of lucent material in the counter-bore andsealed to the crucible.
 12. A light source as claimed in claim 11,wherein the crucible and the plug are of vitreous material and the plugis sealed to the crucible by local melting of the material of the plugat the step and/or the counter-bore.
 13. A light source as claimed inclaim 12, wherein the crucible and the plug are of ceramic material andthe plug is sealed to the crucible by local melting of frit material atthe step and/or the counter-bore.
 14. A light source as claimed in claim11, wherein the plug is flush with the crucible at outer surfacesthereof.
 15. A light source as claimed in claim 11, wherein the sealedplug is recessed and a second plug is provided in the counter-bore flushwith the crucible at outer surfaces thereof.
 16. A light source asclaimed in claim 1, in combination as a lamp with a microwave drivecircuit comprising: a microwave source and a matching circuit.
 17. Alight source as claimed in claim 1, the body within the Faraday cagebeing a resonant waveguide.
 18. A light source as claimed in claim 1,wherein the void is formed by a bore in the body.